Integrated analog video receiver

ABSTRACT

An analog video receiver implemented in an integrated circuit device. The analog video receiver includes an amplifier to amplify an analog video signal having a desired carrier frequency, and a mixing circuit to mix the amplified analog video signal with a complex sinusoid having a frequency substantially equal to the carrier frequency.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of, and hereby incorporates by reference,U.S. patent application Ser. No. 12/191,905, filed Aug. 14, 2008,entitled “Integrated Analog Video Receiver;” which is a division of U.S.patent application Ser. No. 11/120,571 (now U.S. Pat. No. 7,453,526),filed May 2, 2005, entitled “Integrated Analog Video Receiver;” whichclaims priority from the following U.S. Provisional Applications:

Appn. No. Filing Date Title 60/567,191 Apr. 30, 2004 DSP-BASED ULTRALOW-POWER TUNER/DECODER ARCHITECTURE 60/612,108 Sep. 21, 2004 ADAPTIVEPICTURE-QUALITY CONTROL IN MOBILE ANALOG VIDEO RECEIVERS 60/612,282 Sep.21, 2004 RESOLUTION-ADAPTIVE POWER CYCLING FOR ANALOG VIDEO RECEIVERS60/623,193 Oct. 29, 2004 A ZERO-IF ANALOG VIDEO RECEIVER ARCHITECTURE60/634,468 Dec. 9, 2004 ADAPTIVE IMAGE REJECTION CALIBRATION FOR DIRECT-CONVERSION RECIEVERS

FIELD OF THE INVENTION

The present invention relates to the field of analog video reception.

BACKGROUND

Historically, tuner demodulators (“tuner cans”) for video bandapplications have been implemented entirely in the analog domain, usingup to several hundred discrete components and consuming as much as twoto three watts of power. Unfortunately, despite their low cost androbust performance, power and size considerations make discrete tunercans unsuitable for a number of emerging video applications, such asanalog video reception on mobile telephones, personal digitalassistants, laptop computers or other small portable devices.

To meet the demand for small, low-power tuners, designers have begunimplementing tuners in silicon, in most cases with architectures thatmimic the superheterodyne operation of discrete tuner cans;down-converting the carrier frequency of a desired channel to a fixedintermediate frequency (IF), then passing the IF signal through animage-rejection stage to filter spectral components at imagefrequencies. Unfortunately, the notoriously poor performance ofintegrated passives makes it difficult to achieve a satisfactory balancebetween image rejection and power consumption. Consequently existingsilicon tuners typically exhibit either compromised performance at lowpower, or reasonable performance at high power.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings and in whichlike reference numerals refer to similar elements and in which:

FIG. 1 illustrates a direct-conversion analog video receiver accordingto one embodiment;

FIG. 2 illustrates an exemplary embodiment of azero-intermediate-frequency (zero-IF) converter;

FIG. 3 illustrates an exemplary embodiment of an offset canceller;

FIG. 4 illustrates a more detailed embodiment of an offset canceller;

FIG. 5 illustrates a digital embodiment of an offset canceller;

FIG. 6 illustrates in-phase and quadrature waveforms generated by thezero-IF converter of FIG. 2;

FIGS. 7A and 7B illustrates gain and phase errors that may occur in thezero-IF converter of FIG. 2 and the resulting undesired image that mayfold onto the desired signal;

FIG. 8 illustrates an embodiment of an adaptive image rejection filter;

FIG. 9A illustrates an embodiment of the baseband demodulator of FIG. 1;

FIG. 9B illustrates the frequency response of an exemplary filter usedto implement a vestigial sideband compensator within the basebanddemodulator of FIG. 9A;

FIG. 10A illustrates the complex spectrum of a signal received withinthe baseband demodulator of FIG. 9A;

FIG. 10B illustrates the complex signal spectrum that results fromup-conversion within the baseband demodulator of FIG. 9A;

FIG. 10C illustrates the real signal that results from a combination ofin-phase and quadrature signal components in the baseband demodulator ofFIG. 9A;

FIG. 11 illustrates an embodiment of a video decoder that may be used toimplement the decoder 103 of FIG. 1;

FIG. 12 illustrates an embodiment of a power manager that may be used toplace an analog video receiver into a low power mode during intervals inwhich superfluous video information would otherwise be received; and

FIG. 13 is a timing diagram that illustrates the operation of the powermanager of FIG. 12.

DETAILED DESCRIPTION

In the following description and in the accompanying drawings, specificterminology and drawing symbols are set forth to provide a thoroughunderstanding of the present invention. In some instances, theterminology and symbols may imply specific details that are not requiredto practice the invention. For example, the interconnection betweencircuit elements or circuit blocks may be shown or described asmulti-conductor or single conductor signal lines. Each of themulti-conductor signal lines may alternatively be single-conductorsignal lines, and each of the single-conductor signal lines mayalternatively be multi-conductor signal lines. Signals and signalingpaths shown or described as being single-ended may also be differential,and vice-versa. Similarly, signals described or depicted as havingactive-high or active-low logic levels may have opposite logic levels inalternative embodiments. As another example, circuits described ordepicted as including metal oxide semiconductor (MOS) transistors mayalternatively be implemented using bipolar technology or any othertechnology in which a signal-controlled current flow may be achieved.Also signals referred to herein as clock signals may alternatively bestrobe signals or other signals that provide event timing. With respectto terminology, a signal is said to be “asserted” when the signal isdriven to a low or high logic state (or charged to a high logic state ordischarged to a low logic state) to indicate a particular condition.Conversely, a signal is said to be “deasserted” to indicate that thesignal is driven (or charged or discharged) to a state other than theasserted state (including a high or low logic state, or the floatingstate that may occur when the signal driving circuit is transitioned toa high impedance condition, such as an open drain or open collectorcondition). A signal driving circuit is said to “output” a signal to asignal receiving circuit when the signal driving circuit asserts (ordeasserts, if explicitly stated or indicated by context) the signal on asignal line coupled between the signal driving and signal receivingcircuits. A signal line is said to be “activated” when a signal isasserted on the signal line, and “deactivated” when the signal isdeasserted. Additionally, the prefix symbol “/” attached to signal namesindicates that the signal is an active low signal (i.e., the assertedstate is a logic low state). A line over a signal name (e.g., ‘ <signalname>’) is also used to indicate an active low signal. The term“coupled” is used herein to express a direct connection as well asconnections through one or more intermediary circuits or structures. Theterm “exemplary” is used herein to express an example, not a preferenceor requirement.

An integrated analog video receiver is disclosed herein in variousembodiments. In a number of embodiments, the analog video receiverincludes a direct-conversion tuner that converts a broadcast-frequencyanalog video signal directly to baseband, rather than firstdown-converting to a mid-range intermediate frequency. By thisoperation, the image frequency is the frequency of the desired signalitself, so that the large, power-hungry channel selection filtertypically stationed at the intermediate-frequency (IF) output stage of aconventional superheterodyne tuner may be replaced by a significantlysmaller and lower power low-pass filter. In other embodiments, offsetcancellation circuitry is provided to dynamically cancel offsets thatmay result from local oscillator coupling back into the signal input ofthe direct-conversion stage, and an IQ balance circuit is provided toadaptively reject undesired images that may result from phase and gainimbalance in the direct conversion stage. In yet other embodiments, apower management circuit is provided to power down selected componentsof the analog video receiver during intervals in which otherwisesuperfluous video information would be received. These and otherembodiments and aspects of the invention are described in further detailbelow.

FIG. 1 illustrates a direct-conversion analog video receiver 100according to one embodiment. The analog video receiver 100 includes adirect-conversion tuner 101, video decoder 103, audio recovery stage 106and power manager 107, any or all of which may be integrated onto asingle integrated circuit (IC) device, referred to herein as a host IC.The host IC may be a single IC die or an IC package containing two ormore die (e.g., a multi-chip module). Also, the host IC may itself be acomponent of any number of host systems including, without limitation,consumer electronics devices (e.g., television sets, mobile telephones,personal computers, personal digital assistants (PDAs), video players,set-top boxes, etc.), military video-reception systems, televisionsystems provided within various type of transport vehicles (e.g.,automobiles, aircraft, trains, watercraft, etc.) and so forth. The hostsystem may include various types of user-interface for receivinguser-supplied channel selections, configuration information, and thelike, as well as a display to display a video signal recovered by theanalog video receiver 100 and, optionally, an audio transducer togenerate an audible output of an audio signal recovered by the analogvideo receiver 100.

In the embodiment of FIG. 1, the direct-conversion tuner 101 includes aninput-tracking filter 111, a low-noise amplifier 113, a zero-IF (zerointermediate-frequency) converter 115, IQ balance stage 117, basebanddemodulator 119 and offset canceller 121. The input-tracking filter 111is coupled to a signal source 125 (e.g., an antenna or jack forreceiving a cable or other electrically or optically conductive medium)and is tuned to pass a frequency band containing a desired analog videosignal. More specifically, in one embodiment, the input tracking filter111 is a pass band filter tuned in response to a user-specified channelselection to one of a number of video signal channels, each selectablechannel having a different center frequency within a larger spectrum setaside for analog video transmission. In one embodiment, for example, thetransmission spectrum corresponds to a terrestrial broadcast spectrum(e.g., 50-850 MHz), although virtually any frequency range may beencompassed within the transmission spectrum in alternative embodiments(e.g., a cable transmission spectrum from 50 MHz-1 GHz, or any otherspectrum). The selected analog video signal is amplified by thelow-noise amplifier 113 (which may also be tuned to provide gain at theselected pass band), then passed to the zero-IF converter 115. Note thatthe input-tracking filter and/or the low-noise amplifier may be omittedfrom the direct-conversion tuner and implemented, for example, in aseparate integrated circuit device or in a discrete-component circuit.Also, the positions of the low noise amplifier 113 and filter 111 may beinterchanged so that the incoming signal is amplified before beingfiltered and, in some applications, the filter 111 may be omittedaltogether.

In one embodiment, the zero-IF converter 115 is a synchronous detectorthat multiplies the incoming analog video signal by a complex sinusoid,thus down-converting the video signal directly to baseband and producingin-phase (I) and quadrature (Q) baseband signal components that areoutput to the IQ Balance stage 117. In one embodiment, the complexsinusoid is generated at a frequency that matches (or substantiallymatches) the center frequency of the selected pass-band, a frequencyreferred to herein as the carrier frequency of the incoming analog videosignal. It should be noted that, due to the side-band filtering appliedin vestigial side-band (VSB) modulated television signals, the carrierfrequency of the incoming analog video signal may be offset from thefrequency of the original modulated carrier (e.g., offset by F_(C)/2,where F_(C) is the spectral reduction achieved by side-band filtering).

Still referring to FIG. 1, the IQ balance stage 117 compensates for gainand phase error in the zero-IF converter 115 to produce balanced (i.e.,substantially image-free) I and Q component signals, and the basebanddemodulator 119 recovers the final baseband video signal from thebalanced I/Q signals. The offset canceller is provided to compensate foran undesired direct-current (DC) offset that may result from coupling ofa local oscillator within the zero-IF converter back into the receiverinput. IQ balance, baseband demodulation and offset cancellationoperations are all described below in reference to more detailedembodiments.

The audio recovery stage 105 recovers an audio output from an audiocomponent, if any, in the output of the baseband demodulator 119 (oralternatively from the output of the IQ Balance stage 117 or zero-IFconverter 115). For example, in one embodiment, the audio recovery stage105 includes a band pass filter to pass the audio component of thebaseband demodulator output and a demodulator to recover the audiooutput from a frequency-modulated (FM) or amplitude-modulated (AM)digital audio signal. If the audio component has not already beendigitized in earlier stages, the audio recovery stage 105 may alsoinclude an analog-to-digital converter (ADC) and corresponding digitalfilter to generate a filtered digital representation of the audiosignal. An audio trap, not shown in FIG. 1, may be provided in the videopath (e.g., at the input to the video decoder 103) to prevent the audiosignal from reaching downstream video processing stages. The audiorecovery stage 105 may be implemented in various other analog and/orDSP-based circuits (or processes) in alternative embodiments.

Continuing with the video path, the baseband demodulator 119 outputs therecovered baseband video signal to a timing-recovery stage 135 andluma-chroma processing stage 131 within the decoder 103. The luma-chromaprocessing stage 131 extracts chroma (color) information, if any, fromthe baseband video signal and provides corresponding hue (U) andsaturation (V) signals to a format conversion stage 133 along with aluma (intensity) signal (Y) that remains after the color information isextracted. In one embodiment, the format conversion stage 133 is a YUVto RGB converter that converts the incoming intensity, hue andsaturation signals into red, green and blue signals for driving a colordisplay (e.g., a cathode-ray tube (CRT), liquid crystal display (LCD),plasma display, projected display or any other type of image renderingdevice). In alternative embodiments, the format conversion stage 133 mayconvert the YUV components into another signaling format (e.g., YIVsignals), or the formatting stage may be omitted and the YUV signalcomponents used to directly drive the host-system display.

The timing recovery stage 135 (which may be viewed as part of the tuner,rather than the decoder) extracts vertical retrace and horizontalretrace timing signals, referred to herein as v-sync and h-sync signals,respectively, from the tuner-supplied video signal and outputs thetiming signals to the format conversion stage 133 where they are used todelineate frames, fields and scanlines (e.g., v-sync indicating thestart of each video field in a field-interlaced video signal format, andh-sync indicating the start of a scanline to be displayed as a line ofpixels on the display device). In the embodiment shown, the timingrecovery stage may also output timing signals (h-sync, v-sync or both)to the offset canceller 121 and/or power manager 107 to time operationstherein. In one embodiment, discussed in further detail below, the powermanager 107 is used to shut down (i.e., disable or otherwise place in areduced power consumption state) selected components of the tuner 101and decoder 103 during intervals in which superfluous video informationwould otherwise be received. In alternative embodiments, the powermanager 107 may be omitted.

FIG. 2 illustrates an exemplary embodiment of the zero-IF converter 115and its interconnection to the offset canceller 121, low noise amplifier113 and input-tracking filter 111. The zero-IF converter 115 includes alocal oscillator 151, mixer elements 153 a, 153 b (collectively, mixers153), summing circuits 155 a, 155 b, low pass filters 157 a, 157 b, andbuffer amplifiers 159 a, 159 b. In the embodiment shown, the localoscillator 151 is implemented by a phase-locked-loop 161 (PLL), and 90°delay element 163 that generate quadrature sinusoidal signals (e.g.,sine and cosine) at a selected carrier frequency. In a specificimplementation, for example, a divider circuit within the PLL 161divides the output of a voltage-controlled oscillator (VCO) by aselected divisor (e.g., selected according to the center-frequency of auser-specified analog video channel) to generate a closed-loop controlvoltage that establishes the oscillating frequency of the VCO andtherefore the frequency of the sinusoid output from the PLL. Designatingthe PLL output to be a cosine signal, passing the cosine wave throughthe 90° delay element 163 yields a sine wave (i.e., phase offset fromthe cosine wave by a quarter-cycle of the VCO oscillating frequency).The sine and cosine signals constitute quadrature sinusoids at theselected carrier frequency, F_(C), and thus collectively form a complexsinusoid, e^(jωt), where ω=2πF_(C).

The cosine and sine signals generated by the local oscillator 151 aresupplied to mixer elements 153 a and 153 b, respectively, where they aremixed (e.g., multiplied) with the incoming analog video signal togenerate in-phase and quadrature video signals 154 a and 154 b. Themixer elements may be implemented, for example, by four-quadrantmultiplier circuits (also called Gilbert cells) or any other circuitcapable of performing a signal multiplication. The in-phase andquadrature video signals 154 a and 154 b are supplied to low-passfilters 157 a, 157 b which filter out the spectral components at 2F_(C)(i.e., multiplication of sinusoids at frequency F_(C) yields the desireddown-converted signal at baseband (F_(C)−F_(C)), and undesiredcomponents at 2F_(C) (F_(C)+F_(C))), and then supplied to bufferamplifiers 159 a, 159 b to produce amplified I and Q baseband videosignals 170 a and 170 b, respectively. Though not specifically shown,the I and Q signals 170 a and 170 b may be converted to digital signalsin a video ADC stage before being output to the IQ balance stage (i.e.,element 117 of FIG. 1). Also, signals 170 a and 170 b, or either ofthem, may be provided to the audio recovery stage (element 105 ofFIG. 1) before or after conversion to digital form.

DC Offset Cancellation

One challenge presented by the direct conversion tuner 101 of FIG. 1 isthat, as shown in FIG. 2 at 160, the radio-frequency (RF) sinusoidsgenerated by the local oscillator 151 may couple into the sensitiveamplifier of the tuner itself. More specifically, the RF sinusoids aregenerated at the frequency of the selected video channel, andconsequently, if picked up by the signal source (e.g., the antenna) willbe passed by the input-tracking filter, amplified by the low-noiseamplifier, and then mixed with themselves in the mixing elements 153.Unfortunately, a sinusoid mixed with itself yields a DC (direct-current)signal component (e.g., cos²(ωt)=[1+cos(2ωt)]/2 andsin²(ωt)=[1−cos(2ωt)]/2) that may cause buffer amplifiers 159 a, 159 bto saturate or otherwise disrupt operation of downstream receiverstages. The offset canceller 121 is provided to avoid such disruption bycanceling the DC component of the mixer output signals 154 a, 154 b.

In the embodiment of FIGS. 1 and 2, the offset canceller 121 receives ablanking signal from the timing recovery circuit 135 at the start ofeach blanking interval (e.g., horizontal blanking interval and/orvertical blanking interval) and disables reception of the incomingsignal during at least a portion of the blanking interval, for example,by opening a switch 165. While signal reception is disabled, the offsetcanceller samples the mixer output signals 154 a, 154 b to obtain ameasure of the DC offset, if any, then updates (i.e., adjusts) offsetcancellation signal 166 a, 166 b according to the DC offset. The offsetcancellation signals 166 a, 166 b are summed with the mixer outputs 154a, 154 b, respectively, in summing circuits 155 a, 155 b to reduce theDC level of the mixer outputs 154 a, 154 b to a tolerable level (i.e.,cancel the DC component). In alternative embodiments, the offsetcanceller 121 may operate without decoupling the tuner 101 from thesignal source (i.e., switch 165 may be omitted), for example, if ameasure of the DC offset may be obtained during reception of a nominallysteady-state signal (e.g., during horizontal and/or blanking interval ora period in which the video signal is nulled at the transmissionsource).

FIG. 3 illustrates an exemplary embodiment of an offset canceller 201and its interconnection to a summing circuit 155. The offset canceller201 includes an amplifier 203 having an output coupled viasignal-controlled switch 205 to a shunt capacitor 207. The shuntcapacitor 207 effectively operates as a low-pass filter and samplingelement, and may be replaced by various other low pass filter/samplingelement implementations in alternative embodiments. When a blankingsignal 210 (BL) is asserted, switch 205 is closed, enabling theamplifier 203 to charge the capacitor 207 in proportion to the DC levelof the mixer output 154. The charge developed on the capacitor 207constitutes an offset cancellation voltage that is applied to a controlterminal of a current source 209 (e.g., a transistor gate) and thuscontrols the amount of current delivered to summing circuit 155. Byconfiguring the summing circuit 155 such that an increased current fromcurrent source 209 reduces the DC signal level at the summing circuitoutput (i.e., summing circuit 155 effects a subtraction operation), anegative feedback loop is formed in which the amplifier 203 iterativelyupdates the offset cancellation voltage (each iteration occurring in arespective blanking interval) as necessary to cancel the DC component ofthe mixer output 154.

FIG. 4 illustrates a more detailed embodiment of an offset canceller230, summing circuit 240 and their interconnection to adifferential-output mixer cell 225. In the embodiment shown, the mixercell 225 generates a current-mode differential output signal on outputnodes 154 a, 154 b (e.g., as in a Gilbert cell) and is coupled, via passgate switch 227 (“pass gate”), to a signal input source 125. When adifferential blanking signal 250 (BL+/−) is asserted, pass gate 227 isopened, decoupling the mixer cell 225 from the input source 125 and thusenabling the mixer 225 to generate a current-mode error signal 226 thatreflects undesired coupling of the local oscillator, as well as any 1/Fnoise (i.e., noise inversely proportional to the frequency) and anyimbalance in the tuner stages up to and including the mixer cell 225.The summing circuit 240 includes resistive pull-up elements 241 a, 241 bto generate a differential error voltage on the mixer output nodes 154a, 154 b in proportion to the current-mode error signal 226.

The offset canceller 230 includes a differential amplifier 231 havinginputs coupled respectively to mixer output nodes 154 a, 154 b, andoutputs coupled, via pass gates 233 a, 233 b to current-controltransistors 237 a, 237 b. The pass gates 233 a, 233 b are switched to aconducting state in response to assertion of the blanking signal 250 sothat, when the error signal is present on the mixer output nodes 154 a,154 b, the differential amplifier 231 is enabled to drive the gates ofcurrent-control transistors 237 a, 237 b. More specifically, thedifferential amplifier 231 generates a differential offset cancellationvoltage in proportion to the error voltage so that the current-controltransistors 237 a, 237 b are biased to drive a differential, offsetcancellation current (I_(n1), I_(n2)) onto the mixer output nodes 154 a,154 b. The source terminals of the current-control transistors 237 a,237 b are cross-coupled to the mixer output nodes 154 a, 154 b (i.e.,transistor 237 a coupled to output node 154 b, and transistor 237 bcoupled to output node 154 a) to form a negative feedback loop, therebyenabling offset canceller 230 to iteratively adjust the offsetcancellation current in a direction that drives the error signal onmixer output nodes 154 a, 154 b toward a null value (i.e., zerodifferential voltage). In one embodiment, a capacitive element 235 iscoupled between the gate terminals of the current-control transistors237 a, 237 b to maintain the offset cancellation voltage betweenblanking intervals. In alternative embodiments, separate capacitiveelements may be coupled respectively to the gates of thecurrent-controlled transistors 237 a, 237 b (e.g., each capacitiveelement coupled between ground and the gate terminal of a respectivecurrent-control transistor) to maintain the offset cancellation voltage.More generally, any circuit capable of maintaining the offsetcancellation voltage during non-blanking intervals may be substitutedfor capacitive element 235 in alternative embodiments.

Although summing circuit 240 is depicted as a current-mode summingcircuit is in FIG. 4 (i.e., a wired summing junction), other types ofsumming circuits may be used in alternative embodiments including,without limitation, voltage-mode summing circuits (e.g., using anoperational amplifier), digital logic circuits and state machineoperation (e.g., a programmed processor). Also, while the resistiveelements 241 a, 241 b are shown as being included in the summing circuit240, the resistive elements 241 a, 241 b may be disposed in downstreamtuner or decoder stages in alternative embodiments.

FIG. 5 illustrates an embodiment of an offset canceller 270 thatincludes an analog-to-digital converter 271 (ADC), digital filter 273and digital-to-analog converter 275 (DAC). The ADC 271 responds toassertion of a blanking signal 210 by generating a digitized sample ofthe error signal present on node 272, and outputting the error sample todigital filter 273. The digital filter 273 applies the incoming errorsample in a digital filtering operation to generate an updated offsetcancellation value 274. The digital filter 273 may be, for example, aninfinite impulse response (IIR) filter, finite impulse response (FIR)filter or any other type of filter that generates a moving average (orother mathematical combination) of error samples received from the ADC271. The offset cancellation value 274 is provided to the DAC 275 whichgenerates a corresponding analog offset cancellation signal to be summedwith the mixer output 154 in summing circuit 279, thus completing anegative feedback loop. As in embodiments discussed above, the summingcircuit 279 may be a current-mode summing circuit (in which case the DACmay be a current-mode DAC), voltage-mode summing circuit, or any othertype of summing circuit.

Adaptive Image Rejection

FIG. 6 illustrates in-phase and quadrature waveforms ideally generatedby the zero-IF converter of FIG. 2. More specifically, the in-phasecomponent (I) of the received baseband video signal, y(t), includesbaseband images 302 a and 302 b that result from down-converting signalcomponents at ±F_(C) to baseband, along with images 303 a and 303 b at±2F_(C). Similarly, the quadrature component (Q) of y(t) includesbaseband images 304 a and 304 b and 2F_(C) images 305 a and 305 b. Thebaseband images 302, 304 and 2F_(C) images 303, 305 are shown as righttriangles in FIG. 6 to indicate that they may be asymmetric about theirrespective center frequencies, as in the case of VSB modulation.

Assuming that the I and Q mixing paths within zero-IF converter 300 areprecisely balanced (i.e., sinusoids generated by the local oscillator151 are offset by precisely 90° and gains of amplifiers 159 a and 159 bare equal), the algebraic sum of the in-phase and quadrature componentsyields the baseband and −2F_(C) spectral components shown at 310 and312, respectively. That is, the negative frequency contributions to thebaseband components of the I and Q output signals are inversions of oneanother that cancel when summed, leaving only the desired positivefrequency contribution 310. Thus, in ideal zero-IF converter 300,multiplication by a precisely-generated complex sinusoid effectivelydown-converts both the positive and negative frequency components of avideo signal centered at ±F_(C), yielding a desired baseband component310, and a −2F_(C) component 312 that may be removed by low-pass filterstages 157 a and 157 b (although FIG. 6 illustrates mixing with acomplex sinusoid e^(−j2ΠF) ^(c) ^(t), it should be noted that theincoming video signal may alternatively be mixed with sinusoid e^(+j2ΠF)^(c) ^(t) to up-convert the positive and negative frequency componentsof the ±F_(C)-centered video signal, thus yielding a desired basebandcomponent and a +2F_(C) component that may be removed by low-pass filterstages 157 a and 157 b).

Unfortunately, actual circuit implementations generally yield somedegree of phase error in the complex sinusoid and gain imbalance in theoutput gain stages. Referring to FIGS. 7A and 7B for example, a phaseerror, 4, may be viewed as being split between the sine and cosinecomponents of the complex sinusoid (i.e., cos(ω_(C)t+φ/2) and−sin(ω_(C)t−φ/2)) supplied to mixer elements 153 a and 153 b, and a gainerror, 8, may be viewed as being split between amplifiers 159 a and 159b so that the amplifiers 159 a, 159 b yield gains of A+ε/2 and A−ε/2,respectively. As a result of the phase and gain errors, the in-phase andquadrature components of y(t), in effect, leak into one another so that,instead of the idealized cancellation of negative frequencycontributions shown in FIG. 6, some portion of the negative frequencycontribution appears as an undesired image in the IQ output as shown inFIG. 7B. If large enough, the image may produce undesired artifacts inthe output video signal.

In one embodiment, undesired image resulting from phase and gain errorin the zero-IF converter, is corrected by an adaptive image rejectionfilter implemented in the IQ balance stage 117 of FIG. 1. Morespecifically, recognizing that the error signal that yields theundesired image is a scaled complex conjugate of the desired signalx(t), it follows that the received baseband signal, y(t), may beexpressed as a sum of the desired signal x(t) and its conjugate. Thatis, if the desired signal x(t)=I(t)+jQ(t), then the error signal isgiven by β[I(t)−jQ(t)]=βx*(t), where ‘*’ denotes a complex conjugate.Accordingly, the received baseband signal, y(t), may be expressed as:

y(t)=αx(t)+βx*(t)  (1),

where α and β represent the relative proportions of the desired anderror signals and are complex functions of the phase and gain errors asfollows:

α=[(1−ε/2)e ^(−jφ/2)+(1+ε/2)e ^(+jφ/2)]/2

β=[(1+ε/2)e ^(−jφ/2)−(1−ε/2)e ^(+jφ/2)]/2.

Recognizing further that the readily obtainable complex conjugate of thereceived signal, y*(t), contains an x*(t) component, it follows that bysubtracting a properly scaled version of the complex conjugate of thereceived signal from the received signal itself, the error component ofthe received signal may be canceled, leaving a linearly scaled versionof the desired signal. That is:

y*(t)=α*x*(t)+β*x(t), so that

(β/α*)y*(t)=βx*(t)+(ββ*/α*)x(t)  (2).

Now, combining the simultaneous equations (1) and (2) to cancel theerror term β*x(t) yields:

y(t)−(β/α*)y*(t)=(α−ββ*/α*)x(t)=z(t)  (3).

Thus, as shown in expression (3), the error signal may be eliminated bytaking the complex conjugate of y(t), multiplying by the scaling factorβ/α* and then subtracting the result from y(t), leaving z(t), a linearlyscaled version of the desired signal, x(t).

FIG. 8 illustrates an embodiment of an adaptive image rejection filter325 that operates in accordance with expression (3) and that may be usedto implement the IQ balance stage 117 of FIG. 1. The adaptive imagerejection filter 117 includes a conjugate generator 327, cancellationpath 329 and adaptation loop 331. The conjugate generator 327 generatesa complex conjugate, y*(t), of the input baseband video signal, y(t), byinverting the imaginary component of the complex signal. In oneembodiment, the input signal, y(t), has been digitized (e.g., by an ADCat the front end of the IQ balance stage 117 or back end of the zero-IFinverter stage) so that the conjugate generator may simply flip the signbit of the imaginary component of the input signal. Alternatively, in ananalog implementation, the complex conjugate may be generated bytwisting a differential signal-line pair that carries the imaginarycomponent of the incoming signal.

However generated, the complex conjugate is supplied to a multiplier 335within cancellation path 329 where it is multiplied with a β/α* scalingfactor received from the adaptation loop 343, thereby producing thedesired scaled complex conjugate value, (β/α*)y*(t). The scaled complexconjugate value is supplied to an inverting input of summing circuit 337where it is subtracted from the input baseband video signal y(t) toproduce the desired output signal z(t).

The adaptation loop 331 is provided to generate the scaling factor,β/α*, applied within the cancellation path 329. In the embodiment ofFIG. 8, the adaptation loop 331 includes an adaptive filter 341 andsumming circuit 343 coupled in a negative feedback configuration thatattempts to minimize a difference between the input baseband videosignal, y(t) and its filtered complex conjugate K*y*(t). That is, thecomplex conjugate y*(t) is scaled by the filter coefficient (K*) toproduce a filtered conjugate, K*y*(t). The summing circuit 343 receivesthe input signal y(t) and filtered conjugate at non-inverting andinverting inputs, respectively, and thus subtracts K*y*(t) from y(t) toproduce a difference signal 344 that is fed back to the adaptive filter.The adaptive filter iteratively adjusts the filter coefficient, K*, inresponse to the difference signal 344. In one embodiment, for example, aleast-mean-squares (LMS) adaptive update is applied in each iteration asfollows:

K[n+1]=K[n]+μe*[n]y*[n]  (4),

where μ is a scaling factor and e*[n] is the conjugate of the differencesignal 344 for a given update. Iterative application of expression (4)can be algebraically shown to cause K, the conjugate of the filtercoefficient, to converge to:

K=2α*β*/(ββ*+αα*)  (5),

so that the filter coefficient, K* (which may be generated by aconjugate operation in the update equation or through a separateconjugate operation) converges to:

K*=2αβ/(ββ*+αα*)  (6).

Because α is significantly larger than β, expression (6) may besimplified to:

K*=2αβ/(αα*)=2β/α, or twice the scaling factor to be applied in thecancellation path. Accordingly, the filter coefficient, K*, is outputfrom the adaptive filter 341 to divider circuit 345 which divides K* bytwo to yield the β/α* scaling factor applied in the cancellation path329.

Still referring to FIG. 8, it should be noted that instead of a LMSupdate operation, sign-sign LMS updates may be effected in the adaptivefilter 341. More generally, any adaptive update approach or one-time oroccasional calibration operation that will yield the desired scalingfactor may be used in alternative embodiments. Further, while individualcircuit components for performing multiplication, conjugate and summingoperations are depicted in FIG. 8, all such operations or any subsetthereof may be performed in a programmed processor.

FIG. 9A illustrates an embodiment of the baseband demodulator 119 ofFIG. 1. The baseband demodulator 119 is provided to compensate for thevestigial sideband nature of the signal prior to frequency translation,and then frequency-shift the complex signal, z(t), received from the IQbalance stage such that, when the quadrature signal component issubtracted from the in-phase component of the signal, the imaginarycomponent of z(t) is canceled, leaving only the desired, real signalcomponent, r(t). In the particular embodiment shown, the basebanddemodulator includes a vestigial sideband compensator 352 implemented,for example, as a Nyquist filter with the frequency response shown inFIG. 9B. Other compensator structures and filter responses can beutilized in alternative embodiments. After the compensator 352, a firstmixing element 351 is coupled to receive the in-phase component (I) ofthe incoming signal z(t) and a second mixing element 353 is coupled toreceive the quadrature component (Q) of z(t). Assuming that the incomingsignal has the complex spectrum shown in FIG. 10A (i.e., theright-triangle representing the asymmetric 2ω_(s) spectrum of a VSBcentered at zero hertz), and by providing cosine and sine demodulatingsignals at frequency ω_(s) to mix elements 351 and 353, respectively,the z(t) is up-converted by frequency ω_(s) as shown in FIG. 10B. Bysubtracting the ω_(s)-shifted quadrature signal component from theω_(s)-shifted in-phase signal in summing circuit 355 the imaginarycomponent of the complex signal is effectively canceled, yielding thedesired, real video signal, s(t), having the waveform shown at FIG. 10C.In the context of a VSB color television signal, s(t) is adouble-sideband signal having a luma carrier centered at zero hertz andan offset chroma carrier. Note that the local oscillator used togenerate the complex sinusoid within the zero-IF converter may include asecondary divide loop to generate the sinusoids applied within thebaseband demodulator. In the embodiment above, the local oscillator isassumed to be coherent with the received signal, such coherency beingachieved through any number of frequency-tracking techniques (e.g.,carrier recovery). In alternative embodiments, the local oscillator neednot be coherent with the received signal and envelope detection or otherincoherent recovery techniques may be used.

Decoder with Dynamic Frame-Hold

FIG. 11 illustrates an embodiment of a video decoder 400 that may beused to implement the decoder 103 of FIG. 1, and its interconnection toa video buffer 411. The video decoder 400 includes a luma/chromaprocessing stage 131, an optional format decoder 133, a timing recoverycircuit 135 and a signal-to-noise ratio (SNR) detector 407. In oneembodiment, the luma/chroma processing stage 131 includes a luma/chromaseparator 401 and luma/chroma processing circuits 403 and 405 whichdeliver separated chroma and luma signals to the format converter 133.In one embodiment, the luma/chroma separator 401, processing circuits403, 405 and format converter 133 collectively perform a video decodingoperation in accordance with NTSC, PAL or SECAM standards, to yield astandard RGB video output signal (i.e., either NTSC, PAL or SECAM). Inalternative embodiments, the video output signals based on otherstandards, or non-standard output formats may be generated. The timingrecovery circuit 135 operates generally as described above to recoververtical and horizontal synchronization signals from the demodulatedvideo signal, s(t), and deliver the synchronization signals to theformat converter 133 (i.e., to enable delineation of fields andscanlines) and, if necessary to other circuit components with a videoreceiver, such as the offset canceller described above. The SNR detector407 measures the signal-to-noise ratio (SNR) of the demodulated videosignal, s(t), and compares the SNR with a threshold value. If the SNRfalls below the threshold, the incoming signal is deemed too noisy todisplay and the SNR detector 407 asserts a frame-hold signal 408. Thethreshold value may be programmable (e.g., a value recorded within aconfiguration register of the host IC) to enable a user-selected noisetolerance. Also, in alternative embodiments, measures of signal qualityother than SNR may be used, alone or in combination with the SNR, indetermining whether to assert or deassert the frame hold signal.

In the embodiment of FIG. 11, the video buffer 411 includes a framebuffer 413, summing circuit 415 and multiplexer 417. While theframe-hold signal 408 is deasserted, a first field buffer within framebuffer 413 is loaded with scanline information (i.e., pixel data to bedisplayed on a given line of a display device) for an incoming videofield, while scanline information is read out of a second field bufferand passed to the video output 420 via multiplexer 417. When the firstfield buffer is filled, the video buffer 411 begins loading the secondfield buffer while scanline information is read out of the first fieldbuffer, thus effecting an alternating (ping-pong) buffer arrangement.When the frame hold signal 408 is asserted, scanline loading within theframe buffer 413 is halted (i.e., a frame hold condition), and thescanline information generated by the video decoder 400 is summed withcorresponding scanline information previously loaded into the framebuffer 413 to produce a time-averaged picture output. By this operation,the user may be presented with a relatively frozen picture instead ofthe “snow” that generally results from a lost video signal.

Dynamically Disabled Receiver

As discussed above, in reference to FIG. 1, a power management circuit107 may be provided to shut down selected components within the tuner101 and decoder 103 during intervals in which superfluous videoinformation would otherwise be received. For example, in manysmall-screen video display devices, standard-format video signals conveymore information than necessary to drive the display. In such devices,reception of non-displayed video information (superfluous videoinformation from the standpoint of the video device) represents anunnecessary power expenditure, which, in the integrated analog videoreceiver 100 of FIG. 1 may be avoided through operation of the powermanager 107. In one embodiment, the host system may display only onefield per video frame, so that the power manager 107 may disable any orall components within the tuner 101 and/or decoder 103 during theinterval in which the superfluous frame would otherwise be received,thus reducing video receiver power consumption as much as 50% (or nearly50%). In another embodiment, the host system may discard (i.e., refrainfrom displaying) every Nth scanline, so that the power manager 107 maydisable selected components of the tuner/decoder after reception of eachset of N−1 scanlines to save power. The power manager 107 may alsoperiodically disable power during periods of lost signal (e.g., asindicated by the SNR detector 407 of FIG. 11), occasionally enablingoperation of the tuner/decoder to sample the incoming video signal anddetermine whether the signal has been restored.

FIG. 12 illustrates a power manager 440 according to an embodiment thatdisables operation of the tuner and/or decoder (including any or allcomponents thereof) during every other video field interval. That is, asshown in FIG. 13, at every other assertion of v-sync pulse 452, thepower manager asserts a disable signal 446 to prevent reception (andthus save power) of the h-sync pulses 454 and scanline information thatconstitute a video field. In the embodiment of FIG. 12, the powermanager 440 includes a toggle-configured flip-flop 441 (i.e., invertingoutput coupled to data input), saturating counter 443 and logic AND gate445. The strobe input of the flip-flop 441 is coupled to receive v-syncso that the flip-flop output toggles at each v-sync pulse and is thusreferred to herein as the v-sync toggle signal 442 (VST). In oneembodiment, the power manager 440 is designed to enable reception ofeach v-sync pulse 452 (e.g., to enable proper operation of the timingrecovery circuit 135 of FIG. 1) and therefore asserts the disable signal446 for a number of h-sync pulses 454 that correspond to a video field,and then deasserts the disable signal 446 in time to enable reception ofthe next v-sync pulse. (As discussed above, the h-sync pulses 454 may besynthesized by a PLL that receives v-sync at its reference clock inputand divides a VCO output by the ratio of h-sync pulses to v-sync pulsesfor the relevant video standard.) More specifically, the counter 443 isreset to an initial count value in response to a rising edge of thev-sync toggle signal 442 (i.e., the start of a superfluous video field)and then counts up or down toward a terminal count value in response tofollowing h-sync pulses 454. By programming (or hard-coding orhardwiring) the difference between the terminal count and initial countvalues to match the number of scanlines per video field, the counter 443counts up or down during interval in which the superfluous scanlineswould otherwise be received, reaching the terminal count value andasserting a corresponding terminal count signal 444 (tc) when the finalh-sync pulse is counted. The terminal count signal 444 and v-sync togglesignal 442 are supplied to inverting and non-inverting inputs,respectively, of logic AND gate 445. By this arrangement, the low stateof terminal count signal 444 and high state of the v-sync toggle signal442 generated during the superfluous field interval cause AND gate 445to raise the disable signal 446, as shown in FIG. 13 at 458. When theterminal count signal 444 goes high at 460, marking the end of thesuperfluous field interval, logic AND gate 445 lowers the disable signal446 as shown at 462. The v-sync toggle signal 442 is supplied to a resetinput of the counter 443 so that, when the v-sync toggle signal 442 goeshigh at the start of the next superfluous field (e.g., shown at 464),counter 443 is reset to the initial value and thus enabled to countanother field of h-sync pulses 454.

It should be noted that numerous changes may be made to the powermanager 440 without departing from the spirit and scope of theinvention. For example, the power manager may be implemented by aprogrammed processor instead of dedicated circuitry. Also, one or moreconfiguration values may be supplied to the power manager 440 toestablish the number of h-sync pulses to be skipped, thereby enablingpower manager 440 to be configured for operation in accordance withdifferent video standards (e.g., 625 scanline PAL standard vs. 525scanline NTSC standard). Further, as discussed above, rather thandisabling reception of the scanlines of every other video field,reception may be disabled for every Nth scanline.

Electronic Expression of Circuits and/or Processes

It should be noted that the various circuits disclosed herein may bedescribed using computer aided design tools and expressed (orrepresented), as data and/or instructions embodied in variouscomputer-readable media, in terms of their behavioral, registertransfer, logic component, transistor, layout geometries, and/or othercharacteristics. Formats of files and other objects in which suchcircuit expressions may be implemented include, but are not limited to,formats supporting behavioral languages such as C, Verilog, and HLDL,formats supporting register level description languages like RTL, andformats supporting geometry description languages such as GDSII, GDSIII,GDSIV, CIF, MEBES and any other suitable formats and languages.Computer-readable media in which such formatted data and/or instructionsmay be embodied include, but are not limited to, non-volatile storagemedia in various forms (e.g., optical, magnetic or semiconductor storagemedia) and carrier waves that may be used to transfer such formatteddata and/or instructions through wireless, optical, or wired signalingmedia or any combination thereof. Examples of transfers of suchformatted data and/or instructions by carrier waves include, but are notlimited to, transfers (uploads, downloads, e-mail, etc.) over theInternet and/or other computer networks via one or more data transferprotocols (e.g., HTTP, FTP, SMTP, etc.).

When received within a computer system via one or more computer-readablemedia, such data and/or instruction-based expressions of the abovedescribed circuits may be processed by a processing entity (e.g., one ormore processors) within the computer system in conjunction withexecution of one or more other computer programs including, withoutlimitation, net-list generation programs, place and route programs andthe like, to generate a representation or image of a physicalmanifestation of such circuits. Such representation or image maythereafter be used in device fabrication, for example, by enablinggeneration of one or more masks that are used to form various componentsof the circuits in a device fabrication process.

Section headings have been provided in this detailed description forconvenience of reference only, and in no way define, limit, construe ordescribe the scope or extent of such sections. Also, while the inventionhas been described with reference to specific embodiments thereof, itwill be evident that various modifications and changes may be madethereto without departing from the broader spirit and scope of theinvention. Accordingly, the specification and drawings are to beregarded in an illustrative rather than a restrictive sense. In theevent that provisions of any document incorporated by reference hereinare determined to contradict or otherwise be inconsistent with like orrelated provisions herein, the provisions herein shall control at leastfor purposes of construing the appended claims.

1. A video circuit to output a video image corresponding to a selectedchannel which is one of a plurality of channels of a broadcast spectrum,the video circuit comprising: a video receiver to receive an input videosignal having a carrier frequency which is associated with the selectedchannel and to generate a baseband video signal which is representativeof the input video signal, wherein the video receiver includes a tunerto generate the baseband video signal using the input video signalhaving the carrier frequency which is associated with the selectedchannel; a video decoder, coupled to the video receiver, to generate avideo output signal which is associated with the selected channel,wherein the video decoder generates the video output signal using thebaseband video signal; and a video output circuit, coupled to the videodecoder, to generate video data using the video output signal, whereinthe video output circuit includes: a summing circuit to sum datacorresponding to a plurality of video frames of the video output signal,wherein the plurality of video frames are sequential in time, andwherein the video output circuit outputs the video data which includesthe sum of data corresponding to the plurality of video frames.
 2. Thevideo circuit of claim 1 wherein the video output circuit furtherincludes a frame buffer, coupled to the summing circuit, to store dataas a plurality of video frames and wherein the summing circuit sums thedata of a first video frame and data of a second video frame, whereinthe first and second video frames are sequential in time.
 3. The videocircuit of claim 1 wherein the video decoder further includes a detectorto detect a signal quality of the baseband video signal, wherein thevideo output circuit outputs video data which is representative of thesum of data corresponding to the plurality of video frames only when thesignal quality of the baseband video signal falls below a threshold. 4.The video circuit of claim 3 wherein the signal quality of the basebandvideo signal is a signal-to-noise ratio.
 5. The video circuit of claim 1wherein the video receiver, the video decoder and the video outputcircuit are incorporated into or on the same integrated circuit.
 6. Thevideo circuit of claim 1 wherein the video output circuit includes aframe buffer, coupled to the video decoder, to store data as a pluralityof video frames.
 7. The video circuit of claim 6 wherein the framebuffer includes first and second buffers, and wherein the first andsecond buffers store data which is representative of sequential videoframes.
 8. The video circuit of claim 6 wherein the summing circuit iscoupled to the frame buffer to sum data which is representative of aplurality of video frames.
 9. The video circuit of claim 1 wherein thevideo output circuit, in response to a signal which is representative ofa degradation in quality of the baseband video signal, outputs videodata, which is a time-averaged representation of the video outputsignal, as a substitute for data which is representative of the secondvideo frame.
 10. The video circuit of claim 1 wherein the videocircuitry further includes: a detector to detect a signal quality of thebaseband video signal, wherein the detector outputs a frame hold signalwhen the degradation in a signal quality of the baseband video signalfalls below a threshold; and a frame buffer, coupled to the summingcircuit, to store a plurality of video frames.
 11. The video circuit ofclaim 10 wherein the video output circuit further includes: amultiplexer, coupled to the frame buffer and the summing circuit,wherein the multiplexer: (i) in response to a first state of the framehold signal, outputs data of a video frame stored in the frame bufferand, (ii) in response to a second state of the frame hold signal,outputs the data generated by the summing circuit.
 12. A video deviceincluding the video circuit of claim 1 and a video display to display avideo image conveyed in the video data output by the video outputcircuit.
 13. A video device including the video circuit of claim 1, anantenna to receive the input video signal wirelessly, and a videodisplay to display the video image conveyed in the video data output bythe video output circuit.
 14. A method of operating circuitry whichoutputs video data which is representative of a video imagecorresponding to a selected channel which is one of a plurality ofchannels of a broadcast spectrum, the method comprising: generating abaseband video signal, wherein the baseband video signal is a basebandversion of an input video signal; generating a video output signal,which corresponds to the selected channel, using the baseband videosignal; summing video data which corresponds to a plurality of videoframes associated with the video output signal to generate summed videodata, wherein the plurality of video frames are sequential in time; andoutputting video data which corresponds to the summed video data. 15.The method of claim 14 further comprising: detecting a signal quality ofthe baseband video signal; generating a frame hold signal when adegradation in the signal quality of the baseband video signal fallsbelow a threshold.
 16. The method of claim 15 further includingmultiplexing, based on the frame hold signal, between (i) video datawhich is representative of the video output signal and, (ii) the videodata which corresponds to the summed video data.
 17. The method of claim15 wherein the signal quality of the baseband video signal is asignal-to-noise ratio.
 18. The method of claim 14 further comprisingreceiving the input video signal having a carrier frequency which isassociated with the selected channel.
 19. The method of claim 14 whereinsumming video data which corresponds to the plurality of video framesassociated with the video output signal further includes generating atime-averaged representation of the video output signal, wherein thevideo data which corresponds to the summed video data is thetime-averaged representation of the video output signal.
 20. The methodof claim 19 further comprising outputting either (i) the video datawhich is representative of the video output signal or (ii) the videodata which is the time-averaged representation of the video outputsignal.
 21. The method of claim 14 wherein outputting video data whichcorresponds to the summed video data comprises outputting the video datawhich corresponds to the summed video data only when a degradation in asignal quality of the baseband video signal falls below a threshold. 22.The method of claim 14 further comprising displaying a video imageconveyed in the output video data which corresponds to the summed videodata.
 23. A machine readable storage medium having information storedthereon comprising data representing an expression of circuitry tooutput a video image corresponding to a selected channel which is one ofa plurality of channels of a broadcast spectrum, the video circuitrycomprising: a video receiver to receive an input video signal having acarrier frequency which is associated with the selected channel and togenerate a baseband video signal which is representative of the inputvideo signal, wherein the video receiver includes a tuner to generatethe baseband video signal using the input video signal having thecarrier frequency which is associated with the selected channel; a videodecoder, coupled to the video receiver, to generate a video outputsignal which is associated with the selected channel, wherein the videodecoder generates the video output signal using the baseband videosignal; and a video output circuit, coupled to the video decoder, togenerate video data using the video output signal, wherein the videooutput circuit includes: a summing circuit to sum data corresponding toa plurality of video frames of the video output signal, wherein theplurality of video frames are sequential in time, and wherein the videooutput circuit outputs the video data which includes the sum of datacorresponding to the plurality of video frames.
 24. The machine readablestorage medium of claim 23 wherein the expression of circuitry to outputa video image corresponding to a selected channel which is one of aplurality of channels of a broadcast spectrum includes manufacturing asingle integrated circuit including the video receiver, the videodecoder and the video output circuit.